Field of the Invention
Embodiments of the invention generally relate to welding electrodes, and more particularly to an improved welding electrode with low diffusible hydrogen and high Charpy V-notch impact toughness.
Discussion of Related Art
Self-shielded flux cored arc welding (SS-FCAW) has been widely applied to various fabrication industries since the middle of the last century. Its high efficiency and consistent bead quality can significantly reduce the fabrication time and project cost as compared to other welding methods, which is of great importance to fabrication projects in rural areas, such as transcontinental pipeline fabrication and offshore structures installation. On the other hand, due to direct air-exposure, the arc of SS-FCAW can be much harsher with more spattering than a gas-shielding welding arc. The arc disturbance also brings about a negative impact in the control of diffusible hydrogen, oxygen, and nitrogen in the weld metals, and the microstructure evolution and mechanical properties of the welds.
It is known that low diffusible hydrogen content in low carbon steel welds can greatly benefit the cracking resistance and steel fabrication cost and efficiency. Specifically, it is known that minimizing the amount of diffusible hydrogen in deposited weld metal can minimize the possibility of hydrogen related cracking in the resulting weld. Improved self-shielded flux cored wires at the H8 level (i.e., electrodes or electrode-flux combinations capable of depositing weld metal with a maximum diffusible hydrogen content of 8 mL/100 g) have been developed which exhibit good welding performance with a gentle and stable arc, easy-to-command slag flow and robust mechanical properties. Even though the arc performance is designed to be close to a gas-shielding flux cored welding arc, the diffusible hydrogen content of such electrodes still remains at the H8 level, which limits its adoption in fabrication of more critical steel structures.
Prior attempts to reduce the diffusible hydrogen content in low carbon steel welds involved the excessive addition of fluorides to the core mix. The problem with adding fluorides, however, is that it causes fluid slag flow and an unstable arc which may results in failure in out of position welding. It also requires additional aluminum in the core to perform de-oxidation and de-nitridation. As a result, excessive aluminum is retained in the weld, causing grain coarsening and resulting in poor Charpy V-notch impact toughness.
With the increase in high strength steel welding applications, diffusible hydrogen control in weld metal has received increased attention. As a result, there is a need for an improved self-shielded flux cored wire featuring both low diffusible hydrogen content and excellent all-positional welding performance coupled with robust mechanical properties.
In particular, there is a need for an improved self-shielded flux cored wire at or below the 5 ml/100 g level (i.e. the H5 level) that is equivalent to the diffusible hydrogen content achievable using a gas-shielding weld. The improved self-shielded flux-cored welding electrode should also have high Charpy V-notch impact toughness.